Atmosphere for box annealing coated steel



United States Patent 3,111,435 ATMOSPHERE FOR BOX ANNEALENG COATED STEEL Hart Grafi and Arba H. Thomas, Middletown, Ohio,

assrgnors to Armeo Steel Corporation, Middletown,

Ohio, a corporation of Ghio No Drawing. Filed Feb. 21, 1961, Ser. No. 90,614

5 Claims. (=Ji. 14-8127) This invention relates to the production of ferrous metal articles coated with zinc, and relates more particularly to improvements in the methods of low temperature box annealing of Zinc coated steel set forth in copending application Serial No. 844,647, filed October 6, 1959, and entitled Method for Improving the Drawing Quality of Metallic Coated Ferrous Sheet and Strip, now Patent No. 3,028,269, dated April 3, 1962.

In accordance with the teachings of the said co-pending application, it was discovered that the ductility of the ferrous base metal could be materially improved by subjecting the coated stock to a low temperature box anneal following coating. For example, where a ferrous sheet or strip is coated with zinc, it was found that significant improvements in the ductility of the base metal could be achieved by box annealing the coated stock at a temperature of from 450' to 550 F. for a period of from 4 hours up to about 24 hours. The low temperature box anneal was also found to be efficacious in restoring the nonaging properties of normally non-aging ferrous base stock which had been subjected to a heat treatment at 500 and higher as an incident of the coating operation, which heat treatment in part at :least nullified the non-aging properties of the base material. Similarly, rimming steels which are potentially non-aging but which, by reason of a short time heat treatment, have not had their non-aging properties developed, were found to have these properties developed when subjected to a low temperature box anneal following coating.

The teachings of the said co-pending application, which are applicable, with appropriate temperature adjustments, to a relatively wide range of coating metals, inclusive of aluminium, aluminium alloys and terne, have found widespread commercial use in the manufacture of zinc and zinc-iron alloy coated iron and steel. While the low temperature box anneal following coating measurably improves the mechanical properties of the base metal for drawing applications, it was subsequently found that some of the coated stock so produced was unsatisfactory for its intended use by reasonofpoor coating adherence, the coating tending to flake and peel during subsequent drawing operations. While initially it was thought that such defects were caused by loss of temperature control during the box annealing treatment, i.e the elevation of the annealing temperature to a point at which interface alloying or additional alloying would occur, such alloying resulting in the building up of brittle layers, subsequent investigation established that no such undesirable alloying took place. Rather, it was discovered that the pool adherence of the Zinc or zinc-iron coating was caused by interdendritic or intercrystalline corrosion of the coating, and that such corrosion occurred during the low temperature box anneal.

Accordingly, the principal object of the instant invention is the provision of processing techniques which will prevent interdendritic or intercrystalline corrosion of zinc and Zinc-iron alloy coatings during a low temperature box anneal; and in accordance with the instant invention it has been found that such corrosion can be prevented by conducting the box annealing treatment in a controlled atmosphere containing not more than about 0.1% by volume of water vapor. This corresponds to a dew point of approximately 0 F. In this connection, it

may be pointed out that the low temperature box anneal following coating as heretofore practiced utilized a protective atmosphere which was considered to be substantially non-oxidizing to zinc. The most common and widely used protective atmosphere in the steel industry is an essentially nitrogen gas, known commercially as DX, which gas is composed of approximately 9% carbon monoxide, 6% carbon dioxide, 12% hydrogen, and the balance nitrogen, all parts being by volume. Such gas, which is readily available and relatively inexpensive, was initially thought to be highly suited for the low temperature box anneal following coating in that all of its con stituents are considered to be essentially inert with respect to zinc and hence would protect the zinc coating from oxidation during the anneal. Commercial DX gas which has an initial dew point of about 35 F., has been widely used in the commercial box annealing of commodity grades of iron and steel sheets and coils to maintain a bright surface thereon. Under the circumstances it would seem reasonable to assume that such gas would be Wholly satisfactory as a protective atmosphere for the low temperature box annealing of Zinc or zinc-iron alloy coated stock. This seems particularly true since galvanized iron or steel is considered to be highly resistant surface when exposed to all types of weather conditions. Unpain-ted galvanized roofing, gutters, wire, and the like, have been known to resist out-door exposure for decades; and consequently it was extremely surprising to discover that little more than A of 1 percent water vapor in a low temperature heating atmosphere for as short a time as 4 hours would cause interdend-ritic or intercrystalline attack accompanied by severe flaking and peeling of the coating when the coated material is drawn and formed.

In accordance with the instant invention, it has been found that the interdendritic or inter-crystalline corrosion of the coating can be successfully eliminated by conducting the low temperature box anneal in a dry or low dew point atmosphere, which may be either inert or reducing with respect to the Zinc. Essentially, such atmosphere must have two qualifications: (1) it must be non-oxidizing to zinc or zinc-iron alloy-which automatically rules out any free or elemental oxygen containing atmosphere, and (2) it must have a dew point, at the time it enters the furnace, of 0 F. or lower. A negative dew point gas is preferred.

At this juncture it should be pointed out that the zinc ordinarily used for hot dip coatings on steel contains minor amounts of lead, aluminum and other elements. For example, it was discovered a number of years ago that the addition of 0.10.2% aluminum to the zinc coating metal appreciably reduced undesirable alloying between the base metal and the coating, and consequently zinc coatings as they are known today normally include an amount of aluminum in the quantities specified. We have found that such small quantities of aluminum make the zinc coatings particularly susceptible to corrosion during the low temperature box anneal, if a high dew point atmosphere, such as the aforementioned DX gas, is employed. Yet when the aluminum occurs in the zinc only as an impurity, for example, in amounts less than about 0.01%, the zinc or zinc-iron alloy coating is not appreciably attacked when the anneal is conducted in a wet atmosphere having a dew point of F. Furthermore, we have discovered that when the aluminum content of the zinc coating exceeds about 0.7% the coating is also resistant to attack when the anneal is conducted in a wet atmosphere. Therefore, although it is our belief that the corrosive attack on the zinc or zinc-iron alloy coating bears a direct relationship to its aluminum content, the known advantages of having about 0.1 to 0.2% alumimum in the zinc outweigh the disadvantages of lesser eorrosion resistance. By the use of the instant invention, we

have been able to materially improve the properties of the base metal without damaging the properties of the zinc coating. Consequently, the terms zinc" and zinciron alloy as they are used herein and in the claims which follow will be understood to refer to zinc as ordinarily used for hot dip coatings which contain more than about 0.01% aluminum and not more than about 0.7% aluminum.

The gas compositions which may be successfully used in the practice of the invention are many and varied. A commercially available gas which has been found to be highly satisfactory is known as HNX and is composed of approximately 4 to 8% hydrogen and the balance nitrogen. This gas as normally used has a dew point on the order of 40 F. and is essentially inert with respect to zinc and zinc-iron alloys.

Where a zinc coating on steel has been produced, a preferred practice is to box anneal at substantially 45 F. for about 24 hours, or at temperatures up to 550 F. to reduce the soaking time to as little as about 4 hours. Where this is done, the Rockwell B hardness of the coated stock has been reduced from 50 to approximately 40, which indicates a marked softening of the ferrous base material. Where the metallic coating is a Zinc-iron alloy, the box anneal is preferably conducted at a temperature of from 550 F. to 750 F. for a period of at least 4 hours and not substantially more than 24 hours, the temperature for a given zinc-iron alloy being chosen so as to be no greater than that at which the iven zinciron alloy coating will further alloy. It has also been found that temper rolling or heavy roller leveling of the coated material subsequent to coating and prior to the low temperature box anneal will preclude an increase in the ductility of the ferrous base and is to be avoided.

When the foregoing procedures are observed, and the low temperature box anneal is conducted in an essentially dry protective atmosphere as herein disclosed, the resultant zinc or zinc alloy coated products will be capable of deep drawing and the formation of compound curvatures without crazing and cracking or peeling and flaking of the zinc coating.

In addition to the aforementioned HNX gas, other gases which are essentially dry and substantially nonoxidizing to zinc may be employed. Dissociated ammonia (75% H 25 %N may be successfully employed, provided its dew point is at or below about 0 'F. The skilled worker in the art should have no difficulty in ascertaining other protective atmosphere susceptible for use in accordance with the teachings of the instant invention. It is even within the ambit of the instant invention to employ the aforementioned DX gas as the protective atmosphere, provided such gas is first scrubbed or otherwise dried to reduce its moisture content to within the prescribed limit. However, even dried hydrocarbon gases form water by the water-gas reaction at the box annealing temperatures of the invention; and it may be necessary to replace the atmosphere continuously in order to prevent a build-up of moisture above the specified maximum.

Having thus described the invention in certain exemplary embodiments, and with the understanding that modifications may be made therein without departing from 4 the spirit and purpose of the invention, what is desired to be secured and protected by Letters Patent is:

1. In a process for enhancing the ductility of zinc and zinc-iron alloy coated ferrous sheet and strip material wherein the coated material is subjected to a low temperature box anneal following coating, and at the same time preventing interdendritic and interorystalline corrosion of the coating which results in flaking and peeling of the coating during subsequent drawing and forming, the step which comprises conducting said low temperature box anneal in a protective atmosphere which is non-oxidizing to said coating, said protective atmosphere having a dew point no greater than about 0 F.

2. In a process for enhancing the ductility of zinc and zinc-iron alloy coated ferrous sheet and strip material wherein the coated material is subjected to a low temperature box anneal following coating for a period of from 4 to about 24 hours at a temperature of from about 450 to 750 F., and at the same time preventing interdendritic and intercrystalline corrosion of the coating which results in flaking and peeling of the coating during subsequent drawing and forming, the step which comprises conducting said low temperature box anneal in a protective atmosphere which i non-oxidizing to zinc, said protective atmosphere having an initial moisture content no greater than about 0.1% by volume.

3. In a process for enhancing the drawing qualities of zinc and zinc-iron alloy coated ferrous sheet and strip material without impairing the adherence of the coating to the base metal due to interdendritic and intercrystalline corrosion of the coating resulting in flaking and peeling of the coating during subsequent drawing and forming, the steps of subjecting the coated material to a low temperature box anneal following coating, the temperature of the box anneal being below that at which the said coating will alloy with the ferrous base material or form additional alloys not already present on the coated material, said low temperature box anneal being conducted in the absence of temper rolling or heavy roller leveling of the coated material following coating and prior to said box anneal and in a protective atmosphere which is nonoxidizing with respect to said coating, said protective atmosphere having an initial dew point substantially no greater than 0 F.

4. The process claimed in claim 3 wherein said protective atmosphere consists of a gas haying a volumetric composition of from 4 to 8% hydrogen and the balance nitrogen, said gas having an initial dew point of about 40 F.

5. The process claimed in claim 4 wherein the coated material is held at temperature for a period of from about 4 hours to about 24 hours.

References Cited in the file of this patent UNITED STATES PATENTS 2,845,366 Schroeder July 29, 1958 2,859,146 Prust Nov. 4, 1958 3,028,269 Beattie et al. Apr. 3, 1962 OTHER REFERENCES Zinc, The Metal Its Alloys and Compounds, Matherson, published by Reinhold, 1959, Monograph Series No, 142. (Copy in Division 3, page 446 relied upon.) 

1. IN A PROCESS FOR ENCHANCING THE DUCTILITY OF ZINC AND ZINC-IRON ALLOY COATED FERROUS SHEET AND STRIP MATERIAL WHEREIN THE COATED MATERIAL IS SUBJECTED TO A LOW TEMPERATURE BOX ANNEAL FOLLOWING COATING, AND AT THE SAME TIME PREVENTING INTERDENDRITIC AND INTERCRYSTALLINE CORROSION OF THE COATING WHICH RESULTS IN FLAKING AND PEELING OF THE COATING DURING SUBSEQUENT DRAWING AND FORMING, THE STEP WHICH COMPRISES CONDUCTING SAID LOW TEMPERATURE BOX ANNEAL IN A PROTECTIVE ATMOSPHERE WHICH IS NON-OXIDIZING TO SAID COATING, SAID PROTECTIVE ATMOSPHERE HAVING A DEW POINT NO GREATER THAN ABOUT 0*F. 